Method for the elimination in a receiver of incident beams resulting from multipaths, and device for implementation

ABSTRACT

Method for the elimination in a receiver of an incident signal resulting from multipaths, characterized in that an array of 2n auxiliary sensors ( 2, 3 , Ca to Cf), with n≧3, is constructed around an antenna main sensor ( 1 ), these auxiliary sensors all being equidistant from the main sensor and regularly spaced apart, and, by successively taking the pairs of auxiliary sensors symmetric with respect to the main sensor to determine the direction of arrival of the signal resulting from the multipaths, the weighted combination of the signals arising from the n sensors is performed, the direction of the transmitter being known (e jφ     i   ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for the elimination, in areceiver, of incident beams resulting from multipaths, and to a devicefor implementing this method.

2. Description of the Related Art

Systems which nowadays make it possible to strengthen resistance tomultipaths in radionavigation receivers are most of the time based onthe use of more or less particular correlators (narrow correlator,double delta correlator, etc.), making it possible to ignore multipathsdelayed by a greater or lesser distance, always less than twice the chiplength of the pseudorandom code used T, that is to say the period of theclock signals of the codes of the radionavigation signals (for example,for signals of the “Global Positioning System”, GPS type, 1 chip=1 μs inC/A code, i.e. ˜300 m, or 100 ns in P code, i.e. ˜30 m). Nevertheless,for multipaths with small delays relative to the chip length, thesecorrelators turn out to be inefficient.

U.S. Pat. No. 6,175,327 discloses a method for the elimination ofinterference in receivers of GPS radionavigation signals. This methoduses a power inversion technique that calls upon an array of n antennaswhose signals are individually weighted by inversion of across-correlation matrix and use of a canonical constraint notrepresenting a particular reception direction, but a criterion of“minimization of the power received taking account of all the directionsin space”. This known method for eliminating interference is applicableonly to signals whose power referred to each sensor is greater than thelevel of power due to thermal noise. Furthermore, this processing iscarried out without prior knowledge of the direction of arrival of thesignal that one wishes to eliminate. It is therefore impossible withthis known method alone to determine the direction of arrival of one ormore signals due to multipaths, or to eliminate them: their power levelreferred to the input of the receiver is in fact lower than the powerlevel of the main signal (direct path), the power level of which isitself smaller than that of the thermal noise generated in the spectralprocessing band (passband at the input of the receiver).

An object of the present invention is a method making it possible toeliminate on reception a signal due to multipaths and coming from adirection other than that of the transmitter, whatever its delay withrespect to the signal coming directly from the transmitter.

SUMMARY OF THE INVENTION

The method of the invention is a method for determining the direction ofarrival and then for the elimination, in a receiver, of an incidentsignal resulting from multipaths, of received power, in the spectralprocessing band, smaller than that of the thermal noise of the receiver;it is applied to an antenna constructed around an antenna main sensorand comprising an array of auxiliary sensors, these auxiliary sensorsall being equidistant from the main sensor and regularly spaced apart.The method is characterized in that the array of auxiliary sensorscomprises 2n auxiliary sensors, with n≧3, and, that by successivelytaking the pairs of auxiliary sensors symmetric with respect to the mainsensor to determine the direction of arrival of the signal resultingfrom the multipaths, the weighted combination of the signals arisingfrom the n sensors is performed, the direction of the transmitter ofuseful signals being known.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood on reading the detaileddescription of a mode of implementation, taken by way of non-limitingexample and illustrated by the appended drawing, in which:

FIG. 1 is a basic diagram illustrating the processing method inaccordance with the invention, and

FIG. 2 is a diagram showing in slightly greater detail than FIG. 1 themethod of processing the signals arising from the auxiliary sensors, inaccordance with the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Illustrated in a very simplified manner in FIG. 1 is the implementationof the main characteristics of the method of the invention. This methodis applied, in the present case, to radionavigation signals transmittedby a satellite, such as a GPS satellite, forming part of a constellationof positioning satellites, but it is of course understood that theinvention is not limited to this application alone, and that it may beimplemented in other applications, for example the reception of signalstransmitted by terrestrial transmitters, on condition that the directionin which these transmitters are situated with respect to the place ofreception is known.

Represented in FIG. 1 is a main sensor 1 and two auxiliary sensors 2, 3that are aligned with the sensor 1 and disposed symmetrically withrespect to the latter. Each of the sensors 1 to 3 is linked to a complexweighting circuit, respectively 4 to 6. The three circuits 4 to 6 arelinked to a summator circuit 7, at the output 8 of which is gathered asignal S_(-Out) that comprises the useful signal only, that is to say asignal coming directly from the transmitter of the satellite, notaffected by stray signals that are due to multiple reflections of thesignal from the transmitter on various obstacles and that reach thevarious sensors with the useful signal. Represented in FIG. 1 in theform of arrowed half-lines 9 is the direction of arrival of the directpaths of the waves between the transmitter (for example a satellite inthe case of GPS signals) and the sensors 1 to 3. Likewise, representedin the form of arrowed half-lines 10 is the direction of arrival of anindirect path (that is to say after one or more reflections) between thetransmitter of the aforesaid satellite and the same sensors 1 to 3. Themethod of the invention consists in eliminating any beam arriving in thedirection other than the direction of the beams 9, by processing usingthe signals received by the auxiliary sensors. This processing consistsessentially in combining in a weighted manner the signals of theseauxiliary sensors with the signal of the main sensor 1. Thedetermination of the weighting coefficients can be carried out byalgorithms of the “constrained power inversion” type, or according tothe “direct” procedure described in detail below, with reference to FIG.2. In all cases, the method described below for determining thedirection of incidence of the signal arising from multipaths will beused.

Represented in FIG. 2 are the main sensor 1 and two auxiliary sensorsCa, Cf aligned with the sensor 1. These sensors form part of the sixauxiliary sensors Ca to Cf surrounding the sensor 1. All these sensorsare, in the present case, situated on a circle, centered on the sensor 1and regularly spaced over this circle.

Let φ_(i)={right arrow over (k_(direct))}·{right arrow over (1C_(a))}with {right arrow over (k_(direct))} the wave vector of the directincident signal. Let ?_(i)={right arrow over (k_(stray))}·{right arrowover (1C_(a))} with {right arrow over (k_(stray))} the wave vector ofthe stray incident signal (due to multiple reflections of the beam ofthe relevant satellite). The direct signal from the satellite isreceived on the sensor Ca with a phase shift e^(jφ) ^(i) with respect tothe direct signal received on the central sensor 1. The stray signal isfor its part received on the sensor Ca with a phase shift e^(jφ) ^(i)with respect to the stray signal received on the central sensor 1.

As indicated in FIG. 2, the signals of sensors 1 and Ca are correlated(multiplied) at 11 and 12 respectively with the local PRN code (computedin the receiver). The result of the correlation at 11 is endowed at 14with a phase shift e^(jφ) ^(i) and subtracted at 15 from the result ofthe correlation coming from 12. The result of the subtraction at 15gives E_(i). Thus, the value of the expression E_(i)=χ_(i)−e^(jφ) ^(i)·χ_(o), is obtained for each of the auxiliary sensors Ca, Cb, Cc, Cd, Ceand Cf, in which expression χ_(i) is the result of the correlationbetween the code of the signal received on sensor C_(i) (with i rangingfrom a to f) and the local code, χ_(o) being the result of thecorrelation between the code of the signal received on sensor 1 and thelocal code.

We call S_(i) the signal received on sensor C_(i), S_(o) the signalreceived on sensor 1 (the signal received comprises the signal of thedirect beam and the signal of the stray beam), A_(gps) the amplitude ofthe direct signal contained in S_(i), code_(gps) and code_(mul) the PRNcodes of the signal of the direct beam and of the stray beam (these areobviously the same codes phase shifted with respect to one another), (a)the amplitude attenuation factor of the stray signal received during allthe reflections that it has undergone and Ω the overall phase rotationundergone by the stray signal during these same reflexions. We thenobtain:

$\begin{matrix}{E_{i} = {( {S_{i} - {{\mathbb{e}}^{j\;\varphi_{i}} \cdot S_{o}}} ) \otimes {code}_{local}}} \\{= ( {{A_{gps} \cdot ( {{{code}_{gps} \cdot {\mathbb{e}}^{j\;\varphi_{i}}} + {a \cdot {code}_{mul} \cdot {\mathbb{e}}^{j\; O} \cdot {\mathbb{e}}^{j?_{i}}}} )} -} } \\{ {{{\mathbb{e}}^{j\;\varphi_{i}} \cdot A_{gps} \cdot ( {{code}_{gps} + {a \cdot {code}_{mul} \cdot {\mathbb{e}}^{j\; O}}} )} + {noise}} ) \otimes {code}_{local}} \\{= {{( {A_{gps} \cdot a \cdot {code}_{mul} \cdot {\mathbb{e}}^{j\; O} \cdot ( {{\mathbb{e}}^{j?_{i}} - {\mathbb{e}}^{j\;\varphi_{i}}} )} ) \otimes {code}_{local}} +}} \\{{noise} \otimes {code}_{local}} \\{= {{( {A_{gps} \cdot a \cdot {code}_{mul} \cdot {\mathbb{e}}^{j\; O} \cdot {{\mathbb{e}}^{j\;\varphi_{i}}( {{\mathbb{e}}^{j{({?_{i}{- \varphi_{i}}})}} - 1} )}} ) \otimes {code}_{local}} + 0}} \\{= {( {{A_{gps} \cdot a \cdot {\mathbb{e}}^{j\;\Omega} \cdot {\mathbb{e}}^{j(\frac{\psi_{i} + \varphi_{i}}{2})} \cdot 2}\;{j \cdot {\sin( \frac{\psi_{i} - \varphi_{i}}{2} )}}} ) \cdot ( {{code}_{mul} \otimes {code}_{local}} )}}\end{matrix}$

In these expressions, the sign {circle around (×)} represents thecorrelation (or multiplication) operation.

The expression E_(sym(i)) can be obtained likewise by applying the sametype of calculation not to the point C_(i) but to the point C_(sym(i))(for example, if si i=a, sym(i)=f).

If we then evaluate the following expression Ratio_(i):

${Ratio}_{i} = {\frac{E_{i}}{E_{{sym}{(i)}}} \cdot \frac{- 1}{{\mathbb{e}}^{j\;\varphi_{i}}}}$

We obtain:Ratio_(i)=e^(jψ) ^(i)

This signifies that we thus obtain the value e^(jψ) ^(i) of the phaseshift induced by the array of sensors at the location of sensor C_(i),on the path of the stray beam, this amounting to identifying thedirection of incidence of this stray beam.

Next, the signals coming from the n sensors (n=7 in the present case)are combined in a weighted manner, for example a combination byaddition. The weighting coefficient of the signal gathered by thecentral sensor is −(n−1), while the signal gathered by each of theauxiliary sensors (i) is given the coefficient equal to Ratio_(sym(i)).In this way, the signal due to the stray beam is eliminated from thesignal resulting from the weighted combination.

According to a variant of the method of the invention, instead of theweighted combination described above, a method of constrained powerinversion is implemented using fictitiously during the numericalcalculation at input a numerical representation of a very powerfulnarrowband signal in the direction of arrival of the multipath beam. Asmentioned above, a preferred application of the method of the inventionis the processing of signals from radionavigation satellites, whichprocessing requires that seven correlations and weightings be carriedout on the signals received, together with a customary correlation onthe reconstituted signal (after the weighted combinings) so that thisreconstituted signal can be used conventionally by the standard signalprocessing device of the radionavigation receiver used. Of course, thismethod presupposes that the direction of the satellite tracked is known,that is to say that the corresponding values of e^(jφ) ^(i) are known(this being true since almanacs and ephemerides are customarilyavailable giving the trajectories of the satellites transmitting theradionavigation signals). This presupposes that there is a very tightbond between the processing of the antenna signals and theradionavigation tracking processing, this also being true if these twoprocessing operations are performed in the same satellite signalreceiver.

When dealing with signals transmitted by a terrestrial transmitter andlikewise using a method of code measurement type, it is entirelyconceivable to use the procedures cited above insofar as the position ofthe transmitters is known a priori (such is the case for example forfixed transmitting local ground booster stations etc.).

1. A method of eliminating an incident signal resulting from multipathsin a receiver for satellite signals, said incident signal having areceived power, in the processing band, which is smaller than that ofthe thermal noise of the receiver, applied to an antenna constructedaround an antenna main sensor and comprising an array of auxiliarysensors, these auxiliary sensors all being equidistant from the mainsensor and regularly spaced apart, wherein the array of auxiliarysensors comprises 2n auxiliary sensors, with n≧3 of the methodcomprising the steps of determining the direction of arrival of thesignal resulting from the multipaths by successively taking the pairs ofauxiliary sensors symmetric with respect to the main sensor, andperforming the weighted combination of the signals arising from the nsensors, the direction of the transmitter being known, and wherein for anumber n sensors, a weighting by −(n−1) is performed for the mainsensor, and a weighting by Ratio_(sym(i)) is performed for each of theauxiliary sensors of rank i, followed by a summation of all the weightedvalues, with:${{Ratio}_{(i)} = {\frac{E_{(i)}}{E_{{sym}{(i)}}} \cdot \frac{- 1}{{\mathbb{e}}^{j\;\phi_{i}}}}},$in which expression E(i)=(χ_(i)−e^(jφ) ^(i) ·χ_(o)), χ_(i) being theproduct between the code of the signal received on sensor i and thelocal code, χ_(o) being the same product for the main sensor, e^(jφ)^(i) being the phase shift induced on the sensor considered by thedirect signal, E_(sym(i)) being the value homologous to E_((i)) for thesensor symmetric to sensor i with respect to the main sensor andRatio_(sym(i)) the value homologous to Ratio_((i)) for the sensorsymmetric to the sensor i considered with respect to the main sensor. 2.The method as claimed in claim 1, wherein the weighting implements aconstrained power inversion method using fictitiously as starting valuea very powerful narrowband signal that would come from the direction ofarrival of the multipath signal.
 3. A device for the elimination, in areceiver, of incident signals resulting from multipaths, comprising: anantenna main sensor around which is disposed an array of 2n auxiliarysensors, these sensors being disposed on a circle centered on the mainsensor and regularly spaced part, these sensors being linked toweighting circuits and combining circuits, wherein the weighting circuitperforms for a number n of sensors, a weighting by −(n−1) is performedfor the main sensor, and a weighting by Ratio_(sym(i)) is performed foreach of the auxiliary sensors of rank i, followed by a summation of allthe weighted values, with:${{Ratio}_{(i)} = {\frac{E_{(i)}}{E_{{sym}{(i)}}} \cdot \frac{- 1}{{\mathbb{e}}^{j\;\phi_{i}}}}},$in which expression E(i)=(χ_(i)−e^(jφ) ^(i) ·χ_(o)), χ_(i) being theproduct between the code of the signal received on sensor i and thelocal code, χ_(o) being the same product for the main sensor, e^(jφ)^(i) being the phase shift induced on the sensor considered by thedirect signal, E_(sym(i)) being the value homologous to E_((i)) for thesensor symmetric to sensor i with respect to the main sensor andRatio_(sym(i)) the value homologous to Ratio_((i)) for the sensorsymmetric to the sensor i considered with respect to the main sensor. 4.The device of claim 3, wherein the weighting circuit implements aconstrained power inversion method using fictitiously as starting valuea very powerful narrowband signal that would come from the direction ofarrival of the multipath signal.
 5. The device of claim 3, wherein n≧3.6. A method for eliminating signals resulting from multipaths in areceiver for satellite signals, comprising the steps of: determining thedirection of travel of the signal resulting from the mulitpaths, bysuccessively taking pairs of auxiliary sensors symmetric with respect toa main sensor, performing a weighted combination of the signals arisingfrom the sensors, the direction of the transmitter being known, whereinfor a number n of sensors, a weighting by −(n−1) is performed for themain sensor, and a weighting by Ratio_(sym(i)) is performed for each ofthe auxiliary sensors of rank i, followed by a summation of all theweighted values, with:${{Ratio}_{(i)} = {\frac{E_{(i)}}{E_{{sym}{(i)}}} \cdot \frac{- 1}{{\mathbb{e}}^{j\;\phi_{i}}}}},$in which expression E(i)=(χ_(i)−e^(jφ) ^(i) ·χ_(o)), χ_(i) being theproduct between the code of the signal received on sensor i and thelocal code, χ_(o) being the same product for the main sensor, e^(jφ)^(i) being the phase shift induced on the sensor considered by thedirect signal, E_(sym(i)) being the value homologous to E_((i)) for thesensor symmetric to sensor i with respect to the main sensor andRatio_(sym(i)) the value homologous to Ratio_((i)) for the sensorsymmetric to the sensor i considered with respect to the main sensor. 7.The method of claim 6, wherein the weighting implements a constrainedpower inversion method using fictitiously as starting value a verypowerful narrowband signal that would come from the direction of arrivalof the multipath signal.
 8. A processing method in a receiver of anincident signal transmitted in a known direction by a transmitter, forthe elimination of signals arriving on said receiver and resulting frommulti-reflections on various obstacles of said incident signal emittedby the transmitter, said processing method being applied to an antennaconstructed around an antenna main sensor and comprising an array ofauxiliary sensors, these auxiliary sensors all being equidistant fromthe main sensor and regularly spaced apart, wherein the array ofauxiliary sensors comprises 2n auxiliary sensors, with n≧3, a directionof arrival of a signal different from the known direction of theincident signal is determined by successively taking the pairs ofauxiliary sensors symmetric with respect to the main sensor, and aweighted combination of the signals arising from the auxiliary sensorswith the signal arising from the main sensor is performed so that anysignal received which direction of arrival is different from the saidknown direction is eliminated.
 9. A method according to claim 8, whereinthe transmitter is a satellite and the transmitted signal is aradionavigation signal.